1. Field of the Invention
This invention relates to spark timing control methods and spark timing control devices for internal combustion engines, and more particularly to spark timing control methods and devices in which a computing unit computes an optimum spark advance, or spark advance angle, on the basis of at least two operational parameters of an internal combustion engine, e.g., intake manifold vacuum and speed of the engine.
2. Description of the Prior Art
The spark advance angle of an engine may be controlled on the basis of the engine speed and the engine load (the volumetric efficiency of a mixture charge in a cylinder). Of these, the load on the engine is generally detected by measuring the engine's intake manifold vacuum. However, the engine's crank shaft rotates during the given time lag that occurs between the moment an intake manifold vacuum sensor detects an intake manifold vacuum, and the moment a computing unit computes a spark advance angle for a cylinder. It follows that the computing unit does not compute a spark advance angle for the cylinder whose actual volumetric efficiency is measured, but for the succeeding cylinder.
When the load on a cylinder, i.e, the volumetric efficiency of a mixture charge in a cylinder, is low, the spark advance angle should be increased. On the other hand, when the volumetric efficiency of a mixture charge is high, then the spark advance should be reduced. Accordingly, when the load is varied substantially, for instance, during the acceleration, the intake manifold vacuum is immediately increased commensurate to the opening of the throttle valve in the car's carburetor. As a result the spark advance angle for a cylinder of a high volumetric efficiency is allotted to a cylinder of a low volumetric efficiency. Hence, the ignition timing or spark timing used for the cylinder in question is retarded from a desired optimum timing, and the duration of combustion in the cylinder is shortened. This leads to lower torque.
Proposals have been made for an exhaust gas recirculation system which recirculates part of the exhaust gases into an intake system to suppress nitrogen oxides contained in the engine's gases. The spark timing would be controlled by the flow rate of exhaust gases being recirculated. When the exhaust gas recirculation is subjected to a simple on-off control, the spark timing during exhaust-gas recirculation would be advanced as compared with when exhaust-gas recirculation is interrupted. In such a system, when the exhaust gas recirculation is turned "on" from an "off" condition, i.e. when an exhaust gas recirculating valve opens a recirculating passage, the spark timing is immediately advanced. However, even this system produces a given time lag when the recirculating exhaust gases are delivered from the exhaust gas recirculating valve in a recirculating passage to an intake valve in the engine. On the other hand, a time lag exists in terms of crank rotation from the time the cylinder in which recirculating exhaust gases are first filled completes a compression stroke until this cylinder reaches the spark timing condition. Accordingly, the spark timing for a cylinder which is not filled with recirculating exhaust gases is advanced. Hence the cylinder is over-advanced. This results in an increase in the amount of harmful constituents in the exhaust gases.